Cloned determinants for resistance to streptomycin (Sm), kanamycin (Km) and erythromycin (Em), from the streptococcal plasmid pJH1, were used as hybridization probes to establish the presence of these same determinants in 70% of 91 multiple resistant group D streptococci obtained from human and animal sources. The animal isolates had been obtained from 13 states and the human clinical isolates were from the United States, Thailand and Chile. Five plasmids, three from human and two from animal Streptococcus faecalis isolates, although quite different, contained an identical 9 kb segment that included these Sm, Km and Em resistance determinants. One human clinical isolate of S. faecalis was resistant to high levels (greater than 2,000 Mug/ml) of spectinomycin (Sp). The resistance was mediated by a novel plasmid-encoded adenylylating enzyme which modified Sp, but not Sm. A 1.1 kb ClaI/NdeI DNA fragment, carrying the gene for Sp modification, was cloned in and expressed by transformants of Streptococcus sanguis and E. coli. Cloned fragments were used as hybridization probes to identify, in four Sp resistant group D streptococci of animal origin, and 8 kb segment of DNA identical to a region of the Sp resistance plasmid from the human isolate. Preliminary results indicated that the Sp resistance determinant was present in animal strains prior to its emergence in the human S. faecalis strain. Recent clinical isolates of Mycoplasma hominis, resistant to high levels (30 to 100 Mug/ml) of tetracycline (Tc), were shown to contain DNA sequences homologous to the streptococcal Tc resistance determinant, tetM. This was the first evidence for the presence of tetM in an unrelated genus and suggested the spread of tetM from Streptoccocus to Mycoplasma.